The invention relates to a fan, especially for the conveyance of the combustion air in the case of a motor vehicle heater.
Fans of this type, which frequently are ring-duct fans, supply high pressures at relatively low efficiency. Their characteristic curves are steep. Mainly, in the case of higher rotational speed, they are considerably noisy during operation.
Fans of this type are known, for example, from German Pat. No. 902 074 and German Published Patent Application No. 24 09 184. In the case of the known ring-duct fans or lateral-duct compressors, an opening is provided in the area of the impeller between the intake connection and the pressure connection, through which gas can exit, which otherwise would be pulled along from the pressure connection to the intake connection. This measure has the purpose of increasing the efficiency of such known fans. In this case, the gas exiting through the opening may either, by means of a return pipe, be directed to an intermediate point of the lateral-duct (DE-AS No. 24 09 184) or it may also be directed to the intake connection of the compressor of the fan (DE-PS No. 902 074).
The present invention has for a principal object the creation of a fan of the initially mentioned type that has a mechanism for the adjustment of its capacity, which enables the adjustment of the fan output to be performed in a manner that as little noise as possible is developed and an efficiency is achieved that is as high as possible.
According to the present invention, this object is achieved, in accordance with preferred embodiments, by provision of a by-pass duct that is connected with the conveying outlet of the fan and has an adjustable throttling member.
In contrast to the conventional output control, where a throttling member is disposed in the intake duct of the fan, the measure according to the present invention results in the following advantages: The power input of the fan rises with an increasing pressure ratio. In the case of an output by means of the throttling of the intake air, the pressure ratio is increases and, therefore, also the power input. Because of the features of the present invention, however, the pressure ratio is not increased during the reduction of the fan output, because an equalization of pressure takes place through the by-pass duct. Thus, the power input is reduced in comparison to the output control by means of the throttling of the intake air. The development of noise also increases with the pressure ratio. Since, as explained, the pressure ratio is not increased when the output of the fan of the present invention is reduced, the development of noise is also decreased in comparison to the conventional solutions.
When the fan is used in a combustion fan in heaters, the fan according to the present invention also results in the following advantage: When the point in the by-pass duct, where the throttling member is located, is clogged by dirt, this only results in an increase of the fan output. However, an increase of the fan output is harmless in regard to the production of harmful substances (CO.sub.2 and CO). In the case of the conventional solution, on the other hand, where the output control takes place by means of the throttling of the intake air, dirt accumulation at the throttling point would result in a decrease of the combustion air conveyance and, thus, in an increase in the unburned, partially toxic fuel components.
Furthermore, in the case of the damming of the combustion-air intake opening of a heater according to the solution of the state of the art, where the supply of intake air is throttled, the maximally possible fan pressure accumulates at all seals and, in the case of possible small leaks, results in emissions of harmful substances. In the case of the by-pass solution according to the present invention, however, the fan pressure can reach no more than the flow resistance of the by-pass duct (which is significantly lower than the maximal fan pressure).
According to a particularly advantageous aspect of the invention, the efficiency of the fan can be further increased if the air expanding in the by-pass duct is used for admission to the fan impeller. For this purpose, it is provided that the by-pass duct leads into the intake duct of the fan.
Especially in combination with ring-duct fans, the invention is advantageous because ring-duct fans have steep characteristic curves and the adjustment of the output is, therefore, especially critical.
According to an advantageous embodiment, the throttling member in the by-pass duct consists of an adjusting screw. It is especially advantageous to use a setscrew for this purpose.
According to another advantageous embodiment of the invention, this adjusting screw may be disposed so that it also projects into both the intake duct and the by-pass duct so that, to the extent that adjustment of the screw frees the cross section of the by-pass duct, it reduces the cross section of the intake duct. Consequently, the adjustment is especially effective because the two mentioned effects are added to one another.
The efficiency of this control is greater, the closer the by-pass opening is disposed to the lateral duct of the fan, and the more directly the by-pass opening connects the pressure side with the suction of the duct. Accordingly, it is an advantageous feature of the invention to make the by-pass duct between the intake duct and the conveying outlet of the fan as short as possible.
In another development of the preferred embodiment, it is provided that the by-pass duct extends in the proximity of the lateral duct of the ring-duct fan.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.